Re: MPS: weak hash tables

[Eli Zaretskii <eliz@gnu.org>]

> From: Helmut Eller <eller.helmut@gmail.com>
> Cc: Pip Cet <pipcet@protonmail.com>,  Eli Zaretskii <eliz@gnu.org>,  Emacs
>  Devel <emacs-devel@gnu.org>
> Date: Sat, 06 Jul 2024 11:47:58 +0200
> 
> On Sat, Jul 06 2024, Gerd Möllmann wrote:
> 
> >> ert-tests.el worries me the most.  There, MPS looks pretty bad compared
> >> to the old GC.  (Who would have thought that printing backtraces
> >> involves copying and rebalancing interval trees with thousands of nodes?
> >> And why does it perform so poorly with a generational GC?)
> >
> > Hm, sounds almost like a bug to me. The bug being the copying. Why does
> > it do that, do you know?
> 
> One reason may be that something tries to remove invisible text with
> buffer-substring.  That needs to copy part of the interval tree.  But
> the copying happens for both, the MPS and the non-MPS version.
> 
> When I run
> 
>  perf record ../src/emacs -Q --batch -l lisp/emacs-lisp/ert-tests.el
>  --eval '(ert-run-tests-batch-and-exit
>  "ert-test-\\(parse\\|plist\\|record\\|run-tests-batch-exp\\)")'
>  
> then the top entries in the perf output for the MPS version looks like:
> 
>    9.55%  emacs    emacs                       [.] balance_an_interval
>    5.46%  emacs    emacs                       [.] amcSegFix
>    5.20%  emacs    emacs                       [.] exec_byte_code
>    3.71%  emacs    emacs                       [.] _mps_fix2
>    3.10%  emacs    emacs                       [.] fix_lisp_obj
>    2.81%  emacs    emacs                       [.] SegSetGrey
>    2.37%  emacs    emacs                       [.] shieldQueue
>    2.29%  emacs    emacs                       [.] SegFix
>    2.24%  emacs    emacs                       [.] is_dflt_fwd
>    2.07%  emacs    emacs                       [.] alloc_impl
>    2.02%  emacs    emacs                       [.] dflt_scan_obj.constprop.0
> 
> and the non-MPS version:
> 
>   19.63%  emacs    emacs                 [.] balance_an_interval
>   10.22%  emacs    emacs                 [.] exec_byte_code
>    9.73%  emacs    emacs                 [.] process_mark_stack
>    3.04%  emacs    emacs                 [.] next_interval
>    2.42%  emacs    emacs                 [.] sweep_intervals
>    2.26%  emacs    emacs                 [.] Fmemq
>    2.16%  emacs    emacs                 [.] find_interval
>    2.05%  emacs    emacs                 [.] Fcons
> 
> So balance_an_interval is pretty high in both, but it seems that the GC
> related part is quite a bit bigger for the MPS version.  Maybe it has
> something to do with memory barriers.

Isn't the call to balance_an_interval made from sweep_buffers and
sweep_strings?  We balance the intervals of the buffers and strings
that survived GC, to make accessing these trees faster.

How did you conclude that rebalancing interval trees is needed for
printing backtraces?